The laser spark has been used as an excitation source for the in situ analysis of gases, solids, aerosols, and liquids by atomic emission spectroscopy. A powerful laser pulse is focused onto or into the material to be analyzed, thereby vaporizing the material and forming a plasma having high temperature and electron density. By spectrally analyzing light emitted from electronically excited species, one can identify these species.
One problem with this method is that the spark volume and density, and the ultimate temperature of the plasma may vary as a result of sample and laser conditions making quantitative analysis unreliable. Sample conditions that can affect the spark volume and density, and the plasma temperature include the presence of easily ionizable elements such as sodium, the amount of water content, atmospheric pressure, the size and density of microscopic particles and condensed water droplets in gas samples, and the granule size of solid samples, most of which can vary between laser pulses. In addition, the laser pulse energy can vary, which affects the spark size and temperature between pulses.
Another problem with this technique when used for quantitative analysis is a change in the detector efficiency. This can result from drifting optical alignment causing the laser focus to move outside the detector field-of-view, or from deteriorating collection optics causing less light to reach the detector. Both a pulse-to-pulse variation in spark volume and a drift in detector efficiency will cause the intensity of the entire emission spectrum to change, but on different time scales.
Yet another problem with using LIBS for quantitative analysis that has not been considered is that changes in plasma temperature with constant species concentration will cause the laser spark emission intensity and the distribution of intensity among atomic emission lines to change because of changes in the population of higher energy atomic levels. The relationship between spark temperature and the degree of excitation was noticed by Joseph R. Wachter and David A. Cremers in "Determination of Uranium in Solution Using Laser-Induced Breakdown Spectroscopy," Applied Spectroscopy 41, 1041 (1987). Therein, the authors noted that the temperature and electron density of a laser spark in water were found to be 8000 K and 9.times.10.sup.17 cm.sup.-3, respectively, 1 .mu.s after plasma formation, whereas the same quantities for a spark in air were found to be 17,000 K and 2.times.10.sup.17 cm.sup.-3 , and speculated that the spark temperature is likely to be lower in liquids because a large fraction of the laser pulse energy goes into vaporization of the liquid, leaving a smaller fraction for plasma formation, when compared with that for the spark in air. Similar problems exist for the application of other plasma sources to the analysis of gas and solid samples with varying characteristics. For example, a pulsed, electrical discharge spark will have problems of changing plasma temperature and spark volume as sample characteristics change. For continuous plasma sources, such as Inductively-Coupled Plasmas (ICPs) and Microwave-induced Plasmas (MIPs), the steady-state plasma temperature will vary as conditions change, rather than changing between pulses.
The above-described problems may be compensated for in part by introducing known standards into the plasma to calibrate the system. See, e.g., "Detector For Trace Element Analysis Of Solid Environmental Samples By Laser Plasma Spectroscopy, by Richard Wisbrun et al., Anal. Chem. 66, 2964 (1994), where different classes of soil samples containing known amounts of metals were used as calibration standards, and "Metal-Pollution Monitor Passes Field Test," Laser Focus, February 1995, page 16, where it was proposed to introduce certain metals of interest into the waste stream of a smokestack at known levels and monitor the LIBS signals. However, such introduction is generally difficult and unreliable for flowing gases, and sample conditions often change after a calibration is performed both for solids and gases. For quantitative analysis in the laboratory, where gas or other sample characteristics are tightly controlled, calibration does not change and, consequently, the method of using standards in the plasma to calibrate the system works well. Such is not true when attempting quantitative analysis in the field.
Mention has been made of the determination of the temperature for a plasma in thermodynamic equilibrium using the two-line Boltzman method in "Detection of Cadmium, Lead and Zinc in Aerosols by Laser-Induced Breakdown Spectrometry," by Marcelino Essien, Leon J. Radziemski and Joseph Sneddon, J. Anal. Atomic Spectrometry 3, 985 (1988). Therein, it was stated that there is evidence that the laser-induced plasma is in local equilibrium about 1 .mu.s after the onset of plasma formation. Small changes in the laser output and optical alignment were compensated for by the authors by expressing the signal as a ratio of the intensity of the analyte line to the adjacent background. No suggestion or discussion is to be found therein for using the plasma temperature concentration measurements for changing conditions. See also, "Temperature Measurement From First-Negative N.sub.2+ Spectra Produced By Laser-Induced Multiphoton Ionization And Optical Breakdown Of Nitrogen," by Christian Parigger et al., Applied Optics 18, 3331 (1995), and "Electron Number Density And Temperature Measurement In A Laser-Induced Hydrogen Plasma," by Christian Parigger et al., J. Quant. Spectrosc. Radiat. Transfer 53, 249 (1995), for temperature measurements in plasmas. No suggestion or discussion is to be found therein for correcting measurements of line intensities of other species in the plasmas for the plasma temperature during the measurement or for measuring absolute concentrations. For microwave induced plasmas, it is well known that the electron temperature is different from the statistical temperature of the atomic and ionic energy levels. However, this statistical temperature can still be used, since it is that which affects the atomic and ionic emission intensity from specific energy levels. Additionally, a recently published discussion of the "excitation temperature," that is, the electron temperature versus the equilibrium temperature may result in errors in some cases, but not in all cases. See, "Determination Of The Excitation Temperature In A Nonthermodynamic-Equilibrium High-Pressure Helium Microwave Plasma Torch," by M. C. Quintero et al., Appl. Spectro. 51, 778 (1997).
Accordingly, it is an object of the present invention to provide a method for quantitatively determining the concentration of atomic species in gases or solids using a plasma without the uncertainties introduced by variations in temperature and other plasma characteristics.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.